The tRNA specific for methionine (tRNAMet) of human mitochondria contains a formyl‐cytosine at the wobble position of the anticodon to facilitate its binding to AUG, AUA and (in one instance) to AUU. In this issue of The EMBO Journal, Haag et al identify a two‐step enzyme pathway facilitating the modification of the tRNA. Sequential reactions of the methyltransferase NSUN3 and the dioxygenase ALKBH1/ABH1 are important to render the tRNA as able to recognize the non‐canonical methionine codons AUA and AUUs, a property critical for efficient protein synthesis in human mitochondria.
See also: S Haag et al (October 2016)
Eukaryotic cells contain two distinct translation machineries: one in the cytosol and one in mitochondria. While cytosolic ribosomes synthesize thousands of different proteins, the mitochondrial translation system is specialized on the production of only a handful of proteins. The size of the mitochondrial genome, particularly in animals (including humans), was reduced to almost the theoretical minimum. The 16,569 base pairs of human mitochondrial genome code for 13 proteins (all being core constituents of the respiratory chain complexes), two ribosomal RNAs (one forming the core of the large subunit and one the core of the small subunit), and 22 tRNAs (Gustafsson et al, 2016). Except for some small regulatory elements controlling transcription and replication, all other sequences were removed …
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